Re: [PATCH v3 15/16] mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page

From: Vlastimil Babka <vbabka@suse.cz>
To: David Hildenbrand <david@redhat.com>, linux-kernel@vger.kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	David Rientjes <rientjes@google.com>,
	Shakeel Butt <shakeelb@google.com>,
	John Hubbard <jhubbard@nvidia.com>,
	Jason Gunthorpe <jgg@nvidia.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Yang Shi <shy828301@gmail.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Matthew Wilcox <willy@infradead.org>,
	Jann Horn <jannh@google.com>, Michal Hocko <mhocko@kernel.org>,
	Nadav Amit <namit@vmware.com>, Rik van Riel <riel@surriel.com>,
	Roman Gushchin <guro@fb.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Peter Xu <peterx@redhat.com>, Donald Dutile <ddutile@redhat.com>,
	Christoph Hellwig <hch@lst.de>, Oleg Nesterov <oleg@redhat.com>,
	Jan Kara <jack@suse.cz>, Liang Zhang <zhangliang5@huawei.com>,
	Pedro Gomes <pedrodemargomes@gmail.com>,
	Oded Gabbay <oded.gabbay@gmail.com>,
	linux-mm@kvack.org
Subject: Re: [PATCH v3 15/16] mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page
Date: Tue, 19 Apr 2022 17:56:51 +0200	[thread overview]
Message-ID: <b35374d1-858c-393a-7b63-8816efe9b175@suse.cz> (raw)
In-Reply-To: <20220329160440.193848-16-david@redhat.com>

On 3/29/22 18:04, David Hildenbrand wrote:
> Whenever GUP currently ends up taking a R/O pin on an anonymous page that
> might be shared -- mapped R/O and !PageAnonExclusive() -- any write fault
> on the page table entry will end up replacing the mapped anonymous page
> due to COW, resulting in the GUP pin no longer being consistent with the
> page actually mapped into the page table.
> 
> The possible ways to deal with this situation are:
>  (1) Ignore and pin -- what we do right now.
>  (2) Fail to pin -- which would be rather surprising to callers and
>      could break user space.
>  (3) Trigger unsharing and pin the now exclusive page -- reliable R/O
>      pins.
> 
> Let's implement 3) because it provides the clearest semantics and
> allows for checking in unpin_user_pages() and friends for possible BUGs:
> when trying to unpin a page that's no longer exclusive, clearly
> something went very wrong and might result in memory corruptions that
> might be hard to debug. So we better have a nice way to spot such
> issues.
> 
> This change implies that whenever user space *wrote* to a private
> mapping (IOW, we have an anonymous page mapped), that GUP pins will
> always remain consistent: reliable R/O GUP pins of anonymous pages.
> 
> As a side note, this commit fixes the COW security issue for hugetlb with
> FOLL_PIN as documented in:
>   https://lore.kernel.org/r/3ae33b08-d9ef-f846-56fb-645e3b9b4c66@redhat.com
> The vmsplice reproducer still applies, because vmsplice uses FOLL_GET
> instead of FOLL_PIN.
> 
> Note that follow_huge_pmd() doesn't apply because we cannot end up in
> there with FOLL_PIN.
> 
> This commit is heavily based on prototype patches by Andrea.
> 
> Co-developed-by: Andrea Arcangeli <aarcange@redhat.com>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>